1. Field of the Invention
The present invention relates to heat curable, one-component polyurethane reactive compositions for sealing and coating purposes, which have low brushing or spraying viscosities, good longterm flexibility, good overbaking resistance and good adhesion to cathodically electrocoated sheet metal, and which may be cured without blisters and with unimpaired final strength even after storage in an uncured state in humid air.
2. Description of the Prior Art
The use of suspensions of solid polyisocyanates, which have been deactivated by superficial polyadduct formation, in liquid or low-melting compositions containing hydrogen atoms capable of reacting with isocyanates, in particular polyhydroxyl or polyamino compounds, as heat curable adhesive, sealing, coating or embedding compositions and for the production of moldings is known (DE-OS 3,112,054, 3,228,723, 3,230,757, 3,403,499, 3,724,555, 3,919,696, 3,91 9,697).
A common feature of all of these one-component polyurethane reactive compositions is that they may not be stored for very long in their uncured state in humid air without blistering occurring on curing. The blistered structure of the one-component polyurethane reactive compositions arises from the participation of absorbed water in the curing reaction. This blistering is highly undesirable as it impairs the strength of the cured reactive composition and its adhesion. Moreover, if the one-component polyurethane reactive compositions absorb very large quantities of moisture, the actual curing reaction may fail to occur.
In practice, freshly applied adhesive, sealing or coating compositions which have not yet been thermally solidified may sometimes be exposed to humid air for several days before entering the baking oven. This is the case in motor vehicle production when the line is shut down over a weekend. For this reason, curing without blisters and with sufficient strength after relatively extended exposure to humid air is of particular importance to the automotive industry.
While the use of polyether polyamines or the addition of water-absorbing fillers, for example, zeolites, may suppress the water reaction, other disadvantages associated with the use of these formulation components must be accepted. Aliphatic amines are not only costly, but also irritate human skin; aromatic amines are questionable in terms of health; and inorganic fillers such as zeolites very sharply increase the viscosity of the one-component polyurethane reactive compositions. In many cases, zeolites inactivate the added curing catalyst by absorption. While the silane coupling agents described in DE-OS 4,111,654 impart improved resistance to the action of atmospheric humidity to one-component polyurethane reactive compositions, a very large quantity is required.
An object of the present invention is to provide improved one-component polyurethane reactive compositions which have the lowest possible content of polyether polyamines and cure without blisters even after relatively extended storage in humid air.
It has now surprisingly been found that this object may be achieved with the one-component polyurethane reactive compositions according to the present invention, which cure without blisters by the use of certain mixtures of polyether polyols and polyester polyols, even when these compositions have been exposed to relative atmospheric humidity of 55% for three days.
The one-component polyurethane reactive compositions moreover exhibit good resistance to elevated baking temperatures of between 180.degree. and 200.degree. C., in particular when 3,3'-diisocyanato-4,4'-dimethyl-N,N'-diphenylurea is used as the isocyanate component. Softening, decomposition and foaming occur with other isocyanates at these temperatures.